There was just a National Geographic Channel show about this phenomenon this week. There are about 150 twin couples in the US. The interesting thing is that their children are legally cousins, but are genetically siblings.

Now in some places, first cousins can marry. And homozygotic twinning is a genetic trait. Which means each pair of the twin couple could conceivably have twins. If they did, and those twins married, their children would actually be less likely to be exactly like their siblings and cousins than their parents were.

My paternal grandparents were part of a twin couple. My grandmother and great aunt are identical twins and my grandfather and great uncle were identical twins. Their double wedding was a big deal in their town and was in the local newspapers (this was in the late 40s-early 50s.)